Add handler tests and fixture mocks for skyfiles

MERGE REQUEST

MR Guidelines

Overview

This MR started with these goals:

• Remove unnecessary JSON tags from SkyfileMultipartUploadParameters
• Handle invalid DefaultPath that points to a directory instead of a file. This can happen if the skyfile is uploaded via a modified portal.

But then it became apparent that we need some specific testing, so it moved on to add mocking skyfiles via fixtures and this is now the most important aspect of it.

Example for Visual Changes

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods or updated methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to workflow
• Issue added to Sia-UI repo for new supporting features
• Changelog File Created

Issues Closed

Closes #4291 (closed)